1. Field of the Invention
The present invention relates to an electric power steering system.
2. Description of the Related Art
As a first conventional example of a power steering system for a forklift truck, there is known a power steering system in which rear wheels disposed at a rear position of a main body of the forklift truck are used as steering wheels and in which a steering mechanism for operating the steering wheels to turn to thereby turn the vehicle itself and a steering wheel disposed at a front position of the main body of the vehicle are mechanically coupled to each other via a torque transmission mechanism consisting of a chain or a shaft. In this power steering system, the rotational torque of the steering wheel is detected by a torque sensor disposed at a rear end portion of the torque transmission mechanism, and the rear steering wheels are operated to turn as a steering motor provided on the steering mechanism is driven based on a detected value detected by the torque sensor.
As a second example of a conventional power steering system for the forklift truck, there is known a power steering system comprising, as shown in a block diagram of FIG. 5, an operating speed sensor 11 for detecting the operating speed of a steering wheel which is the turning speed thereof, an actual steering speed sensor 12 for detecting an actual operating speed which is an actual turning speed by a steering mechanism, a micro computer 13 which is a controller for calculating a target operating speed which is a target turning speed for the steering mechanism and comparing the actual steering speed detected by the actual steering speed sensor 12 with the target steering speed so calculated for calculation of a deviation and a motor a driving device 15 which is a driving device for driving a steering motor 14 provided on the steering mechanism in response to the deviation so calculated.
As a third example of a conventional power steering system for the forklift truck, there is known a power steering system comprising, as shown in a block diagram of FIG. 6, an operating angle sensor 111 for detecting the operating angle of a steering wheel which is the turning angle thereof, an actual steering angle sensor 112 for detecting an actual operating angle which is an actual turning speed by a steering mechanism or a so-called steering angle of a steering wheel, a micro computer 113 which is a controller for calculating a target operating angle which is a target turning angle for the steering mechanism based on the operating angle detected by the operating angle detecting sensor and comparing the actual steering angle detected by the actual steering angle sensor 112 with the target steering angle so calculated for calculation of a deviation and a motor a driving device 115 which is a driving device for driving a steering motor 114 provided on the steering mechanism in response to the deviation so calculated. In addition, the operating angle of the steering which is the turning angle thereof is calculated in response to the number of times of turns of the steering wheel, and there exists an established proportional relationship between the number of times of turns and the operating angle that the number of times of turns increases as the operating angle increases.
However, in the power steering system according to the first conventional example, since the steering wheel and the steering mechanism are mechanically coupled to each other, it is not easy to change the turning speed ratio between the two members.
Moreover, in the power steering system according to the second conventional example, the target steering speed for the steering mechanism is only calculated based on the simple set ratio relative to the operating speed, and in reality the turning speed ratio between the steering handle and the steering mechanism is set as a fixed value. Namely, in vehicles in general including forklift trucks, since it is important to maintain good straight line vehicle stability by preventing the wandering of a vehicle that occurs while the vehicle is running at high speed, it is the normal practice that the set ratio of the target steering speed of the steering mechanism relative to the operating speed is set as a fixed value by using the straight line vehicle stability at a high vehicle speed as a reference and that the target steering speed of the steering mechanism is calculated as a result of the multiplication of the set ratio by the operating speed.
For forklift trucks as loading vehicles which are different from those developed mainly for running, however, it is imperative to be used to load and unload freight while being run at low speed, and therefore, forklift trucks are operated back and forth restlessly and repeatedly and the steering wheel frequently needs to be turned in a condition in which the vehicle is nearly stationary as in so-called stationary steering. Then, as long as the fixed set ratio set based on the straight line vehicle stability at the high vehicle speed as a reference is used, the turning speed or operating speed of the steering wheel has to be increased in despite of the fact that the target steering speed of the steering mechanism is relatively slow, and therefore there is caused a problem that the operator gets exhausted before long after the initiation of operating the forklift truck or a muscular disorder is caused.
Moreover, in the power steering system according to the third conventional example, the target steering angle for the steering mechanism is only calculated based on the simple set ratio relative to the operating angle, and in reality the turning angle ratio between the steering handle and the steering mechanism is set as a fixed value.
Namely, in vehicles in general including forklift trucks, since it is important to maintain good straight line vehicle stability by preventing the wandering of a vehicle that occurs while the vehicle is running at high speed, it is the normal practice that the set ratio of the target steering angle of the steering mechanism relative to the operating angle is set as a fixed value by using the straight line vehicle stability at a high vehicle speed as a reference and that the target steering angle of the steering mechanism is calculated as a result of the multiplication of the set ratio by the operating speed. Note that in forklift trucks it is common that the number of times of turns of the steering wheel relative to the overall movable range of the steering mechanism is set to 5 to 8 turns.
For forklift trucks as loading vehicles which are different from those developed mainly for running, however, it is imperative to be used to load and unload freight while being run at low speed, and therefore, forklift trucks are operated back and forth restlessly and repeatedly and the steering wheel frequently needs to be turned in a condition in which the vehicle is nearly stationary as in so-called stationary steering. Then, as long as the fixed set ratio set based on the straight line vehicle stability at the high vehicle speed as a reference is used, the number of times of turns of the steering wheel needs to be increased so that the steering angle is also increased in despite of the fact that the target steering angle of the steering mechanism is relatively small, and as a result of this, there is caused a problem that the operator gets exhausted before long after the initiation of operating the forklift truck or a muscular disorder is caused.
The present invention has been made in view of the inconveniences inherent in the conventional examples.
An object of the invention is to provide a power steering system in which a set ratio of a target steering speed for a steering mechanism relative to the steering speed of a steering wheel is variable, and in which there is no risk of the operator of a forklift truck getting exhausted before long after the initiation of the operation of the vehicle or a muscular disorder being caused.
Another object of the invention is to provide a power steering system in which a set ratio of a target steering angle for a steering mechanism relative to the steering angle of a steering wheel is variable, and in which there is no risk of the operator of a forklift truck getting exhausted before long after the initiation of the operation of the vehicle or a muscular disorder being caused.
With a view to attaining the object, according to a first aspect of the invention, there is provided a power steering system comprising a running speed detector for detecting the running speed of a vehicle, an operating speed detector for detecting the operating speed of a steering wheel which is the turning speed of the steering wheel, a controller for calculating a target steering speed which is a target turning speed for a steering mechanism based on the operating speed so detected and comparing an actual steering speed which is an actual turning speed by the steering mechanism with the target steering speed for calculation of a deviation and a driving device for driving the steering mechanism in response to the deviation on so calculated, the controller being configured to change a set ratio of the target steering speed to the operating speed so as to increase or decrease in response to the running speed of the vehicle.
Namely, according to the power steering system, since the set ratio of the target steering speed relative to the operating speed is changed to be increased of decreased in response to the running speed of the vehicle by the controller, a set ratio suitable for a then running speed of the vehicle can be selected. In addition, the target steering speed of the steering mechanism which corresponds to the operating speed of the steering wheel is calculated in consideration of the running speed of the vehicle, and as a result of a deviation being calculated after the comparison of the target steering speed so calculated and the actual steering speed, the vehicle can be operated as required at the operating speed of the steering wheel which corresponds to the running speed of the vehicle or an appropriate turning speed.
According to a second aspect of the invention, there is provided a power steering system as set forth in the first aspect of the invention, wherein the controller is configured to increase the set ratio of the target steering speed to the operating speed when the vehicle is running at low speed and to decrease the set ratio when the vehicle is running at high speed.
According to the power steering system provided with the controller configured as described above, the set ratio of the target steering speed relative to the operating speed increases when the vehicle is running at low speed as in loading or unloading freight, whereas the set ratio of the target steering speed relative to the operating speed decreases when the vehicle is running at high speed as in running with no freight being loaded on the vehicle. As a result, the turning speed of the steering wheel only has to be smaller than the conventional example when the vehicle is running at low speed or the running speed of the vehicle is slow.
According to a third aspect of the invention, there is provided a power steering system comprising a running speed detector for detecting the running speed of a vehicle, an operating angle detector for detecting the operating angle of a steering wheel which is the turning angle of the steering wheel, an actual steering angle detector for detecting an actual steering angle which is an actual turning angle by a steering mechanism, a controller for calculating a target steering angle which is a target turning angle for the steering mechanism based on the operating angle so detected and comparing the target steering angle so calculated and the actual steering angle so detected for calculation of a deviation and a driving device for driving the steering mechanism in response to the deviation so calculated, the controller being configured to change a set ratio of the target steering angle to the operating angle so as to increase or decrease in response to the running speed of the vehicle.
Namely, according to the power steering system, since the set ratio of the target steering angle relative to the operating angle is changed to be increased of decreased in response to the running speed of the vehicle by the controller, a set ratio suitable for a then running speed of the vehicle can be selected. In addition, the target steering angle of the steering mechanism which corresponds to the operating angle of the steering wheel is calculated in consideration of the running speed of the vehicle, and as a result of a deviation being calculated after the comparison of the target steering angle so calculated and the actual steering angle, the vehicle can be operated as required at the operating angle of the steering wheel which corresponds to the running speed of the vehicle or with an appropriate number of times of turns.
According to a fourth aspect of the invention, there is provided a power steering system as set forth in the third aspect of the invention, wherein the controller is configured to increase the set ratio of the target steering angle to the operating angle when the vehicle is running at low speed and to decrease the set ratio when the vehicle is running at high speed.
According to the power steering system provided with the controller configured as described above, the set ratio of the target steering angle relative to the operating angle increases when the vehicle is running at low speed as in loading or unloading freight, whereas the set ratio of the target steering angle relative to the operating angle decreases when the vehicle is running at high speed as in running with no freight being loaded on the vehicle. As a result, the number of times of turns of the steering wheel only has to be smaller than the conventional example when the vehicle is running at low speed or the running speed of the vehicle is slow, thereby making it possible to reduce the steering angle.